H.E.S.S.

High Energy Stereoscopic System

New eyes for the H.E.S.S. I cameras

March 2017

Ten years of dust and duty have not gone unnoticed for the sensitive
cameras on each of the four 12-meter telescopes that comprised the
original H.E.S.S. I array. During this time, there has been the
Galactic plane scan, more
than a hundred source discoveries, a spectrum of
cosmic electrons - a lot
of great measurements. To nobody's surprise, at some point the
endurance of the fragile detectors had started to approach their
limit. In 2012, it was therefore time for the H.E.S.S. collaboration
to place an order with their new collaborators at DESY in Zeuthen to
team up with colleagues from the Paris area, Leicester and Amsterdam
to make use
of the newly developed NECTAr readout chip technology and design new
cameras. Only four years later, in September 2016, the new components
were installed. And now, in the first days of 2017, we received an
alert from our
colleagues in HAWC, and just in time, we
managed to spot the first significant gamma-ray signal found with the
new cameras:

Fig. 1: Gamma-ray sky image of Markarian 421 as seen with
the new H.E.S.S. cameras.

When H.E.S.S. explores the mysteries of the high-energy sky, it
actually does not look into the Universe, but at the upper
Atmosphere. Cosmic gamma-rays are absorbed there and produce short,
faint, violet Cherenkov light flashes that can be detected from the
ground. It requires large mirrors and ultra-fast electronics to record
and digitise them to images that actually display the gamma-ray
incidence. The exposure times per image are as short as 16 nanoseconds
(16/1,000,000,000 of a second), and H.E.S.S. is recording about 300
particle incidences per second. Since some images only consist of a
few handfuls of light particles (photons), the technical requirements
to build such cameras are very challenging.

But the ten years for which the original H.E.S.S. I cameras have been
operated have not only degraded their physical condition; also the
available technologies have developed much further. The internet has
accelerated the development of fast ethernet solutions. Electronic
chips and CPUs have become smaller, faster and capable of doing more
complex tasks. Software has become smarter and faster to develop in an
industrial and standard way. Following these improvements, scientists
have started designing prototypes for the next generation of gamma-ray
observatories. An important technological driver of the next big
experiment in the field, the
Cherenkov telescope array
(CTA), is the NECTAr readout chip. It can capture the fast signals in
Cherenkov cameras an digitise them nanosecond by nanosecond when
requested by the trigger.

This chip, whose complex interior is displayed above, is anticipated
to be used for many CTA mid-size telescopes, a work-horse of the
project, but in fact, it was never used on a telescope before! This
was good reason for the developers of the new H.E.S.S. cameras to take
the opportunity to use this speed-booster for the camera upgrade,which
at the same time could allow verification of the technology for
CTA. We used 16 NECTAr chips on each front-end unit ("drawer"), as can
be seen in this drawer assembly (the square chips with little flowers
printed on them):

But in fact, this is only one of the first-time-ever features of the
project. Because also the project host DESY was relatively new to the
field and had never built a camera for Cherenkov telescopes like
H.E.S.S. before. Still, the engineers, physicists and technicians lost
no time, and with a lot of support from the partnering institutes in
the Paris area (LPNHE, LLR, IRFU), Universities of Leicester and
Amsterdam, MPIK in
Heidelberg and of course endless support by the local MPIK team in
Namibia, the cameras were developed, tested and installed in the short
time frame of only 4 years. The plan foresaw a replacement of all
electronics except the costly photosensors, plus some mechanical
components, like installing a new backdoor to improve the ventilation
in the camera.

On top of this, colleagues from LLR added a full
renewal of the light collimators in front of the PMT pixels ("Winston
cones") to the list of things to improve, so more light is collected
in the first place.

Fig 4a:Installation of one of the cone plates, housing
the new Winston cones, on the front side of the camera.

This does not mean, of course, that there was no room for failure and
funny stories. A deep and gruesome crisis had to be endured to
understand that the new cameras did not harmonise well with the monkey
repelling electric fence that is installed around the H.E.S.S. array
(really, who thinks of that when you're contemplating the detection of
cosmic accelerators?). And big was the adrenaline turnover when the
mechanics learnt that they had designed their first components for a
camera that is one centimetre smaller than they had anticipated. Well,
H.E.S.S. is an experiment, and we like that. At some point, everything
was assembled and working, and the cameras could take off.

In our business, it is a long way though from installation to
operation of a new device. Software needs to be adjusted, network
connections to be established, and a long list of issues that never
occurred in the lab before taught the physicists once more the
difference between theory and practice. Around Christmas 2016,
however, the systems were all fit for observation, and as luck would
have it, an old friend in the gamma-ray sky, the blazar Markarian 421,
was reported in an
Astronomer's Telegram by our other old friends
working with the HAWC detector to have an increased activity in gamma
ray emittance. Or
actually had an increased activity 400 million years ago, which, due
to its distance of 400 million light years, arrived at Earth now and
made it to the news precisely on Jan 4th. Despite it being located in
the Northern sky, in the constellation of Ursa Major, H.E.S.S. turned
its telescopes and had a look. And this is what we saw:

Fig 6: Series of stereoscopic images of candidate gamma-ray
events as seen with then new H.E.S.S. cameras.

Images of particle incidences could be recorded smoothly, and the
source could be detected. The new cameras, which are 4 of only 12
currently operating Cherenkov cameras, seem to work. They deliver the
first large scale demonstration that 3840 NECTAr chips are able to
record gamma-ray signals that are good for Teraelectronvolt
astronomy. And they make us look forward to the final years of
H.E.S.S., where the new cameras, with their much reduced dead time,
and more flexible readout, will provide us with enhanced performance
at both very low and very high energies.